LABILE CARBON APPLICATION AND VINE REMOVAL TO REDUCE NITRATE LEACHING IN SANDY POTATO SYSTEMS

Abstract

Groundwater nitrate (NO3-) contamination is a growing concern in potato (Solanum tuberosum L.) production systems in sandy soils due to high nitrogen (N)-demand crops and coarse texture soil. We tested whether adding labile carbon (C) to soil in the form of glycerol, a biodiesel production byproduct, could decrease NO3- leaching by stimulating microbial activity and increasing N immobilization. Glycerol was broadcast-applied at 0, 1122, and 2244 L ha-1 following potato vine kill at Hancock Research Station, Hancock, WI and Langlade Research Station near Antigo, WI. A similar study was done at a commercial farm in the Wisconsin Central Sands, where glycerol was applied at potato hilling. We used a combination of field sampling, resin lysimeters, and laboratory experiments to assess the effects of glycerol on NO3- leaching, soil NO3- content, and N and C cycling. At Hancock and Antigo, soil NO3- decreased by 33.4 and 101 kg N ha-1 at the high glycerol rate 22.3 and 54.3 kg N ha-1 at the low rate, respectively, compared to the control, but this effect did not last after six weeks at Hancock. Six weeks after glycerol application at Hancock, soil NO3- decreased by 3.4 and 16.5 kg N ha-1 but was not statistically significant. Glycerol did not have a detectable effect on estimated NO3- leaching at Hancock or the commercial farm and there was high variation in NO3- leaching values among resin lysimeters. During all lab incubations soil NO3- was depleted up to 96% regardless of glycerol treatment or sampling timepoint, possibly driven by N immobilization in particulate organic matter. When glycerol was applied to soil in the lab, cumulative 28-d C-mineralization increased by 63% (high glycerol rate) and 29% (low glycerol rate) compared to the control. Although glycerol initially reduced soil NO3-, the larger question is when the immobilized N might reappear. In a separate experiment at Hancock Research Station, we measured the N content of potato vines at the onset of senescence to quantify how much NO3- leaching could be prevented with their removal. Previous work found that removing potato vines from the field can

potentially reduce pathogen pressure, but we are unaware of any research on the impact of vine removal on NO3- leaching in potato systems. In addition to vine removal, we also tested the potential of intercropping with winter rye and banded fertilizer at a reduced rate as strategies to reduce NO3- leaching. Data from resin lysimeters shows that when winter rye was present, estimated NO3- leaching was 35.7 kg N ha-1 less than when no intercrop was present and NO3-leaching tended to be higher under broadcast fertilizer (42 ± 6.7 kg N ha-1) than fertilizer banded at a reduced rate (31.9 ± 5.6 kg N ha-1). The N stock and N percentage of potato vines were not affected by intercropping but were increased by broadcast fertilizer by 11.1% (N percentage) and 22.9% (N stock). The vines had a high average N percentage (1.6 ± 0.03%) and had an average N stock of 38.1 ± 2.0 kg N ha-1, meaning if those vines were removed from the field after desiccation, that amount of leaching could be prevented. Intercropping with rye, banded fertilizer, and vine removal have the potential to at least temporarily reduce NO3- leaching in potato systems, thereby decreasing NO3- inputs to groundwater.